Reduction of contaminants, such as monomers and volatiles, in polymer resins is desirable. One method of reducing the volatile content of (i.e., devolatilizing) such polymers involves increasing the exposed surface area of the resin and accordingly promoting the release of volatiles. An apparatus commonly employed in this operation is known as a devolatilizer nozzle. Examples of devolatilizer nozzles may be found in U.S. Pat. Nos. 5,540,813, 4,294,652, 4,934,433, 5,118,388 and 5,874,525, which are incorporated herein by reference. Such a nozzle may take various shapes, and is generally perforated to permit polymer flow. The nozzle may be hollow such that molten polymer may be pumped into the hollow interior. The pressure of pumping the polymer into the nozzle also forces the polymer through the perforations or holes in the nozzle. The pressure drop across the nozzle associated with forcing molten polymer through the nozzle varies depending on many factors, including polymer viscosity and density, nozzle hole diameter, temperature, flow rate, and nozzle size. The nozzle must be designed to withstand such pressure.
In general, smaller nozzle hole diameters are desirable because they increase devolatilization. On the other hand, pressure on the nozzle may increase as nozzle hole diameters decrease. Thus, to achieve enhanced devolatilization via reduced nozzle hole diameters, the strength and robustness of the devolatilizer nozzle must be enhanced in response to the increased operating pressures.
Higher strength steels may be more resistant to higher pressures, but generally also possess lower ductility and greater hardness, which present manufacturing and reliability issues when producing devolatilizer nozzles. The lower ductility and greater hardness make it more difficult to perforate the metal and roll it into the hoop or circular shape of a nozzle. In addition, the greater hardness of the higher strength steels make them more susceptible to brittle failure.
Thus, improvements in devolatilizer nozzle design that increase devolatilization (i.e., allow smaller hole diameters), account for pressure increases, allow increased production rates, and control manufacturing and material costs are desirable.